This invention relates generally to the field of disc drive data storage devices, or disc drives, and more particularly, but not by way of limitation, to an improved system for mounting the discs in a disc drive.
Disc drives of the type known as "Winchester" disc drives or hard disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM. Higher spindle motor speeds are contemplated for the future.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies or a flex pivot and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
Disc drives of the current generation are included in desk-top computer systems for office and home environments, as well as in laptop computers which are used wherever their users happen to take them. Because of this wide range of operating environments, the computer systems, as well as the disc drives incorporated in them, must be capable of reliable operation over a wide range of ambient temperatures.
Furthermore, laptop computers in particular can be expected to be subjected to large amounts of mechanical shock as they are moved about. It is common in the industry, therefore, that disc drives be specified to operate over ambient temperature ranges of from -5.degree. C. to 60.degree. C., and further be specified to be capable of withstanding mechanical shocks of 100 G or greater without becoming inoperable.
One of the areas of disc drive design which is of particular concern when considering ambient temperature variations and mechanical shock resistance is the system used to mount the discs to the spindle motor. During manufacture, the discs are mounted to the spindle motor in a temperature- and cleanliness-controlled environment. Once mechanical assembly of the disc drive is completed, special servo-writers are used to prerecord servo information on the discs. This servo information is used during operation of the disc drive to control the positioning of the actuator used to move the read/write heads to the desired data location in a manner well known in the industry. Once the servo information has been recorded on the discs, it is assumed by the servo logic that the servo information, and all data subsequently recorded, are on circular tracks that are concentric with relation to the spin axis of the spindle motor. The discs, therefore, must be mounted to the spindle motor in a manner that provides sufficient clamping force to prevent shifting of the discs relative to the spindle motor due to differential thermal expansion of the discs and motor components over the specified temperature range, or due to mechanical shock applied to the host computer system.
Several systems for clamping of the discs to the spindle motor have been described in U.S. Patents, including U.S. Pat. No. 5,452,157, issued Sep. 19, 1995, U.S. Pat. No. 5,274,517, issued Dec. 28, 1993 and U.S. Pat. No. 5,295,030, issued Mar. 15, 1994, all assigned to the assignee of the present invention and all incorporated herein by reference. In each of these incorporated disc clamping systems, the spindle motor of the disc drive includes a disc mounting flange extending radially from the lower end of the spindle motor hub. A first disc is placed over the hub during assembly and brought to rest on this disc mounting flange. An arrangement of disc spacers and additional discs are then alternately placed over the spindle motor hub until the intended "disc stack" is formed. Finally, some type of disc clamp is attached to the spindle motor hub which exerts an axial clamping force against the uppermost disc in the disc stack. This axial clamping force is passed through the discs and disc spacers and squeezes the disc stack between the disc clamp and the disc mounting flange on the spindle motor hub.
From the above description, it would appear that the only element that would need to be considered when designing a disc clamping system would be the disc clamp, with any requirement for additional clamping force being met by an increase in the strength of the disc clamp. However, with the industry trend of size reduction in the overall disc drive, the size of various components within the disc drive has also been reduced, including the thickness of the discs. As the discs have grown thinner, the amount of clamping force that can be applied to the discs without causing mechanical distortion of the discs has also fallen. That is, due to inescapable tolerance variation in the flatness of the disc clamping flange on the spindle motor, the discs themselves and the disc spacers between adjacent discs, as well as the yield strength of the disc material, only a finite amount of axial clamping force can be applied to the inner diameters of the discs before the desired flatness of the disc surfaces is lost.
A need clearly exists, therefore, for a disc clamping system which provides a high resistance to radial shifting of the discs relative to the spindle motor hub, while still maintaining an axial load force that is low enough to prevent mechanical distortion of the discs themselves.
Differences in the flatness due to tolerance variations of the disc clamping flange on the spindle motor, the discs themselves, and the disc spacers between adjacent discs, as well as the yield strength of the disc material is also one of the causes of distortion of discs in a disc stack. The flatness mismatch between the mating surfaces, namely the discs, disc spacer, the spindle hub, and the clamp can cause global distortions on the disc which in turn effects the flatness of the disc. The disc surfaces become cupped or coned, saddle shaped, or even become distorted in higher modes such as potato chipping. As the disc stack is assembled, the flatness of the discs are distorted. When potato chipped, the disc actually becomes wavy at the inner diameter and the disc's shape resembles a potato chip. These distortions emerge as the disc stack is assembled and the mating surfaces of the disc attempt to accommodate the differences in flatness of the other components of the disc drive.
Any distortion due to clamping is undesirable. The slider and transducer do not maintain a constant fly height in the presence of distortions. For example, when the disc distorts to resemble a potato chip the fly height varies. A valley causes the slider and attached transducer to fly low while a hill causes the slider and attached transducer to fly high. In the presence of a coning or cupping type disc distortion, the fly height is too high on one side of the disc and too low on the other side of the disc. The fly height may be constant but will vary from the nominal, designed fly height.
Ideally, the height between the transducer and disc should be uniform. When disc distortions occur on a disc, the fly height varies and the data channel must compensate for the variation in the signal from the transducer. The variation in fly height is more of a problem when a magneto-resistive ("MR") head is used as the read element in a disc drive. Disc distortions also press the limits of the allowable head disc spacing margin and may affect the reliability and capacity of the disc drive.
As a result, there is a need for a disc clamp and clamping method which minimizes disc distortion while also minimizing the amount of height needed to accommodate the clamp. There is also a need for a disc that can be used that will accommodate some of the tolerance mismatches of the other components of the disc stack while minimizing the disc distortions which effect the flatness of the data surfaces of the disc or discs. In other words there is a need for a disc which will allow for better control of the disc geometry that results from assembling the disc into a disc stack. There is also a need for a disc and disc stack in which the tolerances of the spacing rings and clamps could be made less stringent so that the cost associated with the various components would be less. In addition, there is a need for a manufacturing process that requires less stringent tolerances on the clamping force, so as to ease the cost of assembly.